In LTE (Long Term Evolution) having been standardized in 3GPP (3rd Generation Partnership Project) as the 3.9-generation radio transmission system, single carrier frequency division multiple access (SC-FDMA) is adopted for uplink (mobile station to base station) transmission.
In SC-FDMA, high frequency efficiency is achieved by flexible allocation of signal spectra depending on channel quality of individual users. Further, a plurality of users accessing at the same time form a virtual macro antenna array so as to spatially multiplex transmit signals from individual users, whereby MU-MIMO (Multi-User Multiple Input Multiple Output) that can be expected to improve frequency efficiency is supported on the uplink in LTE.
By the way, in the uplink transmission, in order to demodulate transmit signals from individual mobile stations, the base station needs to estimate channel information between each mobile station and the base station with high precision. It is possible to estimate channel information based on the reference signal previously determined between the base station and each mobile station. However, it is impossible to estimate channel information directly when spatial multiplexing by MU-MIMO is performed because the reference signals transmitted from individual users are spatially multiplexed and received.
However, in LTE, spatial multiplexing by MU-MIMO is permitted between the users that perform communications having the same signal bandwidth and the same frequency band (FIG. 12(a)), only. In this case, by using a cyclic shift (CS) technique, shown in non-patent document 1, which gives a cyclic shift unique to the user in the time domain to an identical reference signal sequence that has been given beforehand to the MU-MIMO participating users, each of MU-MIMO participating users could be made to maintain orthogonality of the reference signal with other users. However, since spatial multiplexing by MU-MIMO is permitted for only the users that perform communications having the same signal bandwidth through the same frequency band, there also occur limitations on the frequency scheduling that is determined depending on the channel conditions, so that there was a limit to improve frequency efficiency.
At present, as a predominant candidate of the 4th generation radio transmission system, LTE-A (LTE-Advanced) was proposed, and its standardization has been actively implemented. The uplink transmission in LTE-A is supposed to adopt a transmission scheme of a single carrier basis similarly to LTE. By the way, in the uplink MU-MIMO of LTE-A, in order to further improve frequency efficiency, as distinct from LTE, execution of spatial multiplexing in only part of signal spectra between LTE-A users has also been proposed (FIG. 12(b)).
In this case, a simple application of a CS technique cannot maintain orthogonality between reference signals, so that it is impossible to realize such an uplink MU-MIMO transmission system that permits part of signal spectra to be spatially multiplexed. It is therefore inevitable to establish a new channel estimation method.
There have been other discussions up to now about the methods of estimating channels based on reference signals that have been spatially multiplexed when LTE-A users perform spatially multiplexing in only part of signal spectra in the above way. Non-patent document 2 refers to a method of performing frequency division multiplexing (FDM) using a reference signal having a comb teeth-like signal spectrum, and use of the FDM technique enables channel estimation even when signal spectra of LTE-A users are spatially multiplexed in part.
For example, non-patent document 2 also refers to a method of code division multiplexing (CDM) by performing orthogonal coding of the reference signal in the time direction. In this method, even in the LTE-A system in which signal spectra are spatially multiplexed in part, allocation of multiple reference signals in the time direction enables estimation of channels of individual users that are spatially multiplexed.